>> What are the metaphysical problems of an old earth?>>I supposed the primary one is the need to explain death prior to the fall>of man.

There are several ways of approaching this. The suggestion that that
animal death occurred before the Fall goes back at least to Augustine, and
other authors before the 1700's, so it is not necessarily associated with
belief in an old earth. Biblical reasons for this suggestion include the
need for Adam and Eve to understand the warning, which would be difficult
if nothing died; the command to multiply, which would be unnecessary if
there were no attrition; and the generally spiritual rather than physical
meaning of death through Scripture. Death as we know it is certainly
affected by our fall, but even Enoch underwent some change.

Fossil evidence for predation goes back to the latest Precambrian; fossil
animals go back a bit further in the Late Precambrian (i.e., dead but no
evidence of eating each other).

Possible explanations include:
Satan was already fallen; he may have meddled with creation at an earlier
point. Adam and Eve were assigned work to do in subduing the earth, so
there was something "fixable", at least in the sense of improvement.

The death of animals is not necessarily morally wrong. Buckland wrote
rather exhaustively on this point; it also comes up in Lewis' The Problem
of Pain.

The effects of our fall are not necessarily temporally limited to after our
fall. What Jesus did nearly 2000 years ago affected people from Adam to
ourselves, so the relationship between time and supernatural causes is
complex.